<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text">
<input type="button" value="平方" onclick="f()">
<input type="button" value="求和" onclick="f1()">
<div>结果:</div>
<script>
    let d=document.querySelector("div");
    let i=document.querySelector("input");
    function f1(){
        //d.innerText="结果:"+(i.value*1+i.value*1);
        d.innerText="结果:"+(parseFloat(i.value)+parseFloat(i.value));
    }
    function f(){
        //判断用户输入的内容是否是数值  如果是NaN代表不是数值
        if (isNaN(i.value)){
            d.innerText="输入错误!"
            return;
        }
        d.innerText="结果:"+i.value *  i.value;
    }
</script>
</body>
</html>